1. Field of the Invention
The present invention relates to a digital signal processing apparatus and a communication system using the same, and more particularly, to a digital signal processing apparatus that can comply with a plurality of different low bit rate coding methods, and a communication system using the same.
2. Description of the Background Art
Recently, the development of a digital mobile communication system employing a digital data transmission method, such as automobile telephone systems and portable telephone systems, is in progress in Japan, the United States, and Europe. In such a digital mobile communication system, the transmission rate is improved by transmitting a digital audio signal that is encoded in a low bit rate manner between either a base station connected to a public telephone line or an ISDN line and each terminal equipment coupled to the base station via a radio communication line.
For example, in accordance with the standard of Japanese digital automobile telephone system, the data transmission efficiency between the base station and each terminal equipment is improved by low bit rate coding (digital to digital converting) a digital signal having a transmission rate of 64 kbps into a digital signal having a transmission rate of 11.2 kbps using VSELP (Vector Sum Excited Linear Prediction) technique. In order to realize such data transmission, a digital audio signal processing apparatus is provided at both the base station side and at each terminal equipment side for low bit rate coding a digital audio signal that is to be transmitted, and for decoding a received low bit rate encoded digital audio signal.
FIG. 1 is a block diagram schematically showing the structure of an audio processing section of a cellular automobile telephone system using the above mentioned VSELP technique as an example of a digital mobile communication system. Transmission from a base station side to the terminal equipment side will be described with reference to FIG. 1. A .mu.-law PCM signal of 64 kbps as a digital audio signal supplied from a public telephone line (PSTN) or an ISDN line is subjected to a predetermined multiplexing process by a mobile communication exchange station 1 to be provided to a radio communication line control station 2.
More specifically, this mobile communication exchange station 1 carries out exchange between a mobile communication network and a fixed communication network (PSTN or ISDN) or another mobile communication network. For that purpose, the mobile communication exchange station 1 controls a plurality of a radio communication line control stations 2. The mobile communication exchange station 1, which includes a home memory for storing location information as to each mobile terminal equipment, searches for a mobile location of each terminal equipment in response to calling from the fixed communication network by using the home memory and carries out exchange with the radio communication line control station 2 corresponding to the mobile location.
The communication between mobile communication exchange station 1 and each radio communication line control station 2 is carried out through a high speed digital line. If a digital line of 8 Mbps is used, for example, a plurality of message channels, each of which is formed of digital data of 64 kbps obtained by converting analog audio signal as well as control data, are multiplexed for communication.
Radio communication line control station 2 includes two data multiplexing and separating circuits 21 and 23, and m (m is a positive integer) digital audio signal processing apparatuses 22a, 22b, . . . , 22m provided in parallel according to the number of message channels between these circuits. Data multiplexing and separating circuit 21 separates the multiplexed 64 kbps digital audio signal supplied from mobile communication exchange station 1 for respective message channels to provide single corresponding demultiplexed 64 kbps digital audio signals to digital audio signal processing apparatuses 22a, 22b, . . . , 22m. Each of the digital audio signal processing apparatuses 22a, 22b, . . . , 22m carries out low bit rate coding of the applied 64 kbps digital audio signal of each channel into a 11.2 kbps digital audio signal (digital to digital conversion) to provide that 11.2 kbps low bit rate coded signal to data multiplexing and separating circuit 23. Data multiplexing and separating circuit 23 time-divisionally multiplexes the 11.2 kbps digital audio signals provided from the m digital audio signal processing apparatuses 22a, 22b, . . . , 22m to provide the result to n (n is a positive integer) radio base stations 3a, 3b, . . . , 3n.
In the case of a mobile communication system of the cellular method, the plurality of radio base stations 3a, 3b, . . . , 3n are individually provided in respective corresponding cells. Each of radio base stations 3a, 3b, . . . , 3n digitally modulates the time-division multiplexed digital audio signal and control data supplied from radio communication line control station 2 to transmit that multiplexed signal to a plurality of terminal equipment, that are currently moving in the corresponding cell, via corresponding one of antennas 4a, 4b, . . . , 4n.
As a result, a radio communication line shown by an arrow is established between a terminal equipment 6 installed in a automobile 5 that is moving in each cell and a corresponding radio base station (for example station 3b). Terminal equipment 6 contains a digital audio signal processing apparatus (not shown) equivalent to each of the above-described digital audio signal processing apparatuses 22a, 22b, . . . , 22m for digital-demodulating the time-division multiplexed digital signals supplied from a corresponding radio base station, e.g., station 3b, and then separating the digital audio signal of 11.2 kbps of the corresponding message channel and decoding (digital to digital converting) a resulting separated digital audio signal into a digital audio signal having the original transmission rate of 64 kbps.
The transmission from each terminal equipment to the base station will be described with reference to FIG. 1 hereinafter. A digital audio signal of 11.2 kbps, that is low bit rate encoded by the aforementioned digital audio signal processing apparatus not shown, is time-division multiplexed with the control data in the predetermined format to be digital-modulated and transmitted from terminal equipment 6 side. A radio communication line indicated by an arrow is established between the terminal equipment 6 and a radio base station (for example station 3b) corresponding to the cell in which terminal equipment 6 is moving. Radio base station 3b receives digital signals from a plurality of terminal equipment in the cell via an antenna 4b to digital-demodulate the received signals. The demodulated digital audio signals and control data of a plurality of channels are multiplexed and then provided to radio communication line control station 2.
Digital multiplexing and separating circuit 23 in radio communication line control station 2 separates the digital audio signals of 11.2 kbps from the multiplexed data received from a plurality of radio base stations 3a, 3b, . . . , 3n for respective message channels to provide corresponding demultiplexed digital audio signals to digital audio signal processing apparatuses 22a, 22b, . . . , 22m. Each of digital audio signal processing apparatuses 22a, 22b, . . . , 22m decodes (digital to digital converts) the applied 11.2 kbps digital audio signal of each channel into a digital audio signal of 64 kbps to provide a resulting decoded digital signal to data multiplexing and separating circuit 21. Data multiplexing and separating circuit 21 multiplexes the 64 kbps digital audio signals provided from m digital audio signal processing apparatuses 22a, 22b, . . . , 22m provides the result to a public telephone line or an ISDN line via mobile communication exchange station 1.
FIG. 2 is a block diagram schematically showing a structure of the digital audio signal processing apparatus 22a shown in FIG. 1. Each of the remaining digital audio signal processing apparatus 22b, . . . , 22m has the same structure as that of this digital audio signal processing apparatus 22a, and their description will not be repeated.
At the time of transmission from the base station side, a digital audio signal of 64 kbps (.mu.-law PCM signal) provided from data multiplexing and separating circuit 21 of FIG. 1 is applied to an echo canceler 8 via a terminal 7. Echo canceler 8 removes the echo components in the digital audio signal generated at a 2 line-4 line conversion unit (well known and not shown) included in the public telephone line network. The digital audio signal provided from echo canceler 8 is supplied to a well known digital signal processing circuit (high speed DSP) 9 such as MOTOROLA DSP56116 for converting a digital audio signal having a transmission rate of 64 kbps and a digital audio signal having a transmission rate of 11.2 kbps to each other. A signal processing program for digital to digital conversion by the VSELP technique is stored in a memory (ROM) 10 to be executed by digital signal processing circuit 9. Digital signal processing circuit 9 converts (low bit rate encodes) a digital audio signal of 64 kbps to a digital audio signal of 11.2 kbps in accordance with the program stored in memory 10 to provide the obtained signal to data multiplexing and separating circuit 23 (FIG. 1) via a terminal 11. In the apparatus of FIG. 2, each of terminals 7 and 11 is shared for input and output for simplification of description.
At a time of reception at the base station side, digital signal processing circuit 9 converts (decodes) a digital audio signal of 11.2 kbps supplied from data multiplexing and separating circuit 23 of FIG. 1 into a digital audio signal of 64 kbps in accordance with the program stored in memory 10 to provide the obtained signal to data multiplexing and separating circuit 21 (FIG. 1) via echo canceler 8 and terminal 7. The above-described digital mobile communication system is disclosed in, for example, "Design Concept of Speech Processing Equipment for Digital Land Mobile Communication Systems" 2-411 of National Conference of IECE, Spring 1991.
Although highly efficient data transmission can be carried out in the above-described digital mobile communication system of VSELP method by low bit rate coding a digital audio signal into a transmission rate of 11.2 kbps, there may be a case where the transmission rate is further reduced and the number of communication slots increased to further improve the transmission rate. More specifically, a low bit rate coding method employing a different transmission rate other than the low bit rate coding method employing the aforementioned transmission rate of 11.2 kbps can be considered.
For example, the low bit rate coding using the transmission rate of 11.2 kbps (full rate) has been adopted by the current standard of the digital cellular system, so that signals of three channels are multiplexed to be transmitted through the radio communication line having the transmission rate of 42 kbps. On the other hand, there is a plan of further reducing the transmission rate to 5.6 kbps (half rate) in the future, so that six channels can be multiplexed at the maximum.
Therefore, during the period for changing from full rate to half rate, there may be a case where digital audio signals by low bit rate coding methods of a plurality of different transmission rates are mixed in one digital mobile communication system. In such a case, a plurality of digital audio signal processing apparatus (for example, 22a, 22b, . . . , 22m of FIG. 1) must be provided for each low bit rate coding method, i.e. for each transmission rate, resulting in significant increase in the size of the apparatus at the base station. Furthermore, the manufacturing cost will increase.
It is often difficult from the standpoint of structure to add a digital audio signal processing apparatus corresponding to a low bit rate coding method of a different transmission rate after provision of a digital audio signal processing apparatus corresponding to a low bit rate coding method of the transmission rate of 11.2 kbps, for example, in the base station, and there is a possibility of the original communication service being interrupted in the worst case.